Ilya Yashin holds a report based on work carried out by Russian opposition leader Boris Nemtsov before he was killed.
Photograph: Alexander Zemlianichenko/AP

Colleagues of Russian opposition leader Boris Nemtsov have released a report on Russian soldiers secretly fighting in Ukraine that he had worked on before he was killed in February.

The 65-page report – called Putin. War – details the takeover of Crimea by Russian troops in February 2014 and their subsequent deployment to aid separatist fighters in eastern Ukraine. According to the publication, at least 150 Russian soldiers were killed there in August and at least 70 soldiers were killed helping pro-Russia rebels take the strategic town of Debaltseve in February. Low-level fighting has continued in the region despite a February ceasefire.

The real casualties are likely to be higher, but these numbers were the only dead the report’s authors could confirm, Nemtsov’s friend and fellow activist Ilya Yashin said after a press conference presenting the report.

“We want people to start asking questions of the politicians representing them,” Yashin said. “We want to change public opinion.”

Yashin later tweeted that the site on which the report had been posted, putin-itogi.ru, had undergone a “powerful denial-of-service attack”. The site was again accessible from Moscow on Tuesday after a brief disruption.

“Nemtsov believed that it was possible to explain to Russians what is actually happening, not what the propaganda that’s pouring from the television screen says,” said the politician’s assistant, Olga Shorina.

Nemtsov started gathering information for the report shortly before he was shot in front of the Kremlin on 27 February. The activists said they wanted investigators to consider his work on the report as a possible motive for the assassination.Although none of the report had been written when he was killed, the authors used the table of contents, handwritten notes and documents he had left behind to form the basis of the publication.

Although the report uncovers little new information, it is perhaps the most extensive compilation yet of evidence of the Kremlin’s involvement in the eastern Ukraine conflict, which has killed more than 6,200 people since April 2014. Besides the human cost, the war has resulted in sanctions that have isolated Russia politically and created a refugee crisis, the authors said. In the first 10 months of the war, Russia spent 53bn roubles on deploying troops, sending military equipment and paying the salaries of “mercenaries” fighting on the side of separatists, according to the publication.

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“It’s not a report with sensational information, it’s just a report so that Russian citizens can put everything together to understand the war in Ukraine and to make their own choice – that’s what this report is intended to do,” said former prime minister Mikhail Kasyanov, who led the RPR-Parnas party with Nemtsov.

“This report was prepared by Russian patriots. The war that Putin started with Ukraine contradicts Russia’s interests, it is harming Russia’s interests, and we will do everything we can to stop it,” said Yashin.

In a chapter about the takeover of Crimea, which Putin at first attributed to local self-defence forces, the report quotes Russian soldiers who took part in the military operation to capture key infrastructure on the peninsula in February 2014.

In August, regular Russian troops and military equipment allowed separatists to turn the tide against a Ukrainian offensive, the report said. Russian forces again fought alongside the rebels in the battle of Debaltseve, a strategic town that was captured from the Ukrainian military in February, shortly after a ceasefire agreement had been signed by the warring sides. The soldiers were forced to officially resign from the army before they were deployed and throw away identifying markings so that they could pose as rebels.

The report’s authors met with the relatives of soldiers who had fought in eastern Ukraine, but they refused to go on the record because they “feared for their lives,” according to Yashin. In addition, the families of the 70-plus servicemen killed in Debaltseve, who had officially resigned before deployment, haven’t received obligatory compensation payments, according to the report.

Instead, it catalogues evidence such as the capture and questioning of 15 Russian soldiers in Ukraine on several occasions in August. It also lists the names of servicemen confirmed to have been killed there. In one instance, a screenshot shows that the wife of a soldier from the Pskov region asked friends on a Russian social network to come to his funeral in August, before deleting the post.

According to the report, one of whose authors was economist and former deputy minister of finance Sergei Aleksashenko, the average salary for a rank-and-file separatist fighter in eastern Ukraine was 60,000 to 90,000 roubles a month. It also detailed the presence of equipment like the Tornado S multiple rocket launcher, which is not exported outside of Russia. In addition, members of the security forces of Chechen leader Ramzan Kadyrov were proven to be in eastern Ukraine, it said.

Moscow has spent at least 80bn roubles since last year supporting more than half a million Ukrainian refugees who fled to Russia. The report also estimated that the rising inflation exacerbated by the western sanctions over Russia’s involvement in the conflict has impacted wages and savings, costing the country’s population trillions of roubles.

Yashin said the authors hoped to gather funding to print more than the current 2,000 copies. The report is also available at Putin-itogi.ru. They had trouble finding someone willing to print the publication and have received threats related to their activism, he added. Nemtsov’s previous reports on state corruption, including one on the preparations for the 2014 Winter Olympics in Sochi, have not been covered by state media.

“We will continue this work despite the pressure on us now and the pressure that may be put on us in the future,” Yashin said. “They can pressure and intimidate us, but they can’t shut our mouths.”

Irina Vorobyova, a pensioner and activist with the opposition group Solidarity, was waiting outside the press conference to obtain a copy of the report. She said she had handed out copies of Nemtsov’s previous reports to ordinary Russians on the street.

“For them, there’s no one but Putin, but we’ve always handed out these reports before,” she said. “Some people say, ‘get that away from me’ and don’t take it, but others are more positive.”

Putin’s spokesman Dmitry Peskov said on Tuesday he had not seen the report and was not sure if he would read it, Interfax news agency reported.